Leading edge flaps, such as variable camber Krueger (VCK) flap systems, are an attractive means of low speed stall protection on modern aircraft due to their relative simplicity and low weight. In order to meet the specific aerodynamic requirements of new high speed wing designs which have evolved in recent years, it is necessary that the flap produce good lift coefficients for both landing and take-off operations. For example, unlike most prior generation aircraft, in which the take-off position was not critical, the wings proposed for many aircraft under development will be critical in all flying conditions; consequently, for these new aircraft to obtain the desired performance, the leading edge flap system must provide (a) high lift coefficients with minimum drag for take-off, (b) the highest possible lift coefficient for the slowest possible landing, and (c) a small radius leading edge and minimum drag for efficient cruise. For a VCK-type leading edge flap system to provide these characteristics, it must (a) be configured with sealed, or near sealed, trailing edges for takeoff, (b) be configured to provide a sufficiently large trailing edge slot for landing, and (c) be stowed with a sufficiently small radius leading edge for cruise.
A number of VCK flap systems and other devices have been proposed for changing the camber of an airfoil to improve low speed handling, while preserving suitable cruise characteristics. For example, a VCK leading edge flap has been incorporated in the Boeing 747-type airplane, and a number of flaps of this and similar types are shown in the patent literature, examples of these being given below.
U.S. Pat. No. 4,262,868 (Dean) shows a VCK flap which has the three operating positions discussed above. This device utilizes a three-point support for the variable camber flap surface, The trailing edge of the flap is supported off the wing by a small bell crank 33 which is connected to a chordwise extending beam 24; the rearward end of this beam is connected to the rear edge of the flap panel and the forward end is pivotally connected to the leading edge. The middle portion of the panel is supported from the central portion of the beam by a linkage. The beam is connected to the lower arm of a drive crank, the upper arm of which is connected to the support hell crank mentioned above. The drive crank extends the flap forwardly, and as this is done, links 16c and 37 push the middle portion of the flap panel outwardly to give it more camber. A separate bullnose 12 also rotates into position against the leading edge of the flap panel for take-off and landing.
While the Dean system may be suitable for many applications, it exhibits drawbacks in terms of cost, Weight and efficiency. First, because the device employs only a relatively simple pivoting motion (primarily about points 34, 17, and 15) to move between its deployed positions, it is not possible for it to provide the ideal positioning and angular orientation of the flap in each of these locations: in the landing position the panel should be relatively steeply angled to the airflow and have its trailing edge positioned forward of the leading edge of the wing to form the aerodynamic slot, while in the take-off position the panel should be more shallowly angled and have its trailing edge moved more or less directly rearwardly to the leading edge of the wing to form the aerodynamic seal. However, the Dean linkage cannot provide the motions necessary to do this, and so the flap's actual location and angulation in these positions represents a less than ideal compromise. Also, the Dean system utilizes three separate supports along the flap panel, as well as an additional support for the separate bullnose, and each of these is expensive to fabricate and adds weight. Furthermore, because the bullnose piece is separate from the flap panel, an aerodynamically inefficient discontinuity is created between these during deployment. Still further, because the support bell crank 33 is positioned quite close to the leading edge of the wing, this interferes with the normal installation of anti-icing ducting in this area.
U.S. Pat. No. 3,743,219 (Gorges) shows another 3-position VCK flap system. In this case, the rear edge of the flap panel is connected directly by a link 64 to a bell crank 60, link 64 being suspended from the wing by a rocker arm 74. A pair of links connected to the other arm of the bell crank form a scissors arrangement which foreshortens the distance between the leading and trailing edges of the flap so as to bow the flap outwardly to the desired camber. The skin thickness of the flap is tapered, and this determines the proper amount of camber when flexed and also the position at which the greatest amount of camber occurs. While this arrangement has the advantage of being simple, it, like the Dean linkage, is unable to generate the required motions to ideally locate and orientate the flap in its two operative positions. Furthermore, because it relies on the memory of the panel material to achieve the desired camber, and also provides only one point support for the bullnose, the cambered flap is subject to instability due to aerodynamic forces. Still further, since the rigid bullnose cannot bend back upon itself, it is difficult to stow compactly.
U.S. Pat. No. 3,504,870 (Cole, et al. '870) shows another VCK flap system having a stowed position under the wing and a high lift landing position in which a slot is formed with the leading edge of the Wing. However, this device does not have the third position in which the trailing edge of the flap forms a seal with the wing for take-off. The linkage utilizes three separate support locations along the flap panel, and a separate bullnose piece, and so shares disadvantages with the Dean system. The rear edge of the flap panel is connected to a link 24 which is moved forwardly and rearwardly by the middle portion of a crank arm 16. The lower end of the crank arm is connected to a link 28 which acts through another link 39 to rotate the bullnose, and this also acts through a link 32 to move the middle portion of the flap panel outwardly to increase its camber.
The following patents share generally the disadvantages of the devices discussed above, notably the inability to achieve an ideal angulation and positioning of the flap for both landing and take-off.
U.S. Pat. No. 3,941,334 (Cole '334) is directed to a variable camber airfoil where the camber of the leading edge can be changed to increase or decrease lift. This incorporates a flap assembly which is generally similar to that shown by Cole et al. '870. U.S. Pat. No. 3,904,451 (Cole '451) discloses essentially the same apparatus as Cole '334.
U.S. Pat. No. 3,910,530 (James, et al.) shows another leading edge flap which is moved between stowed and deployed positions by means of a linkage having an arm 56 which is actuated by bell crank. At an intermediate pivot location 60, the arm 56 has a direct connection to the flap, by which it moves the flap to and from its deployed position. An outer pivot connection 88 of arm 56 acts through a link 86 to deploy a separate bullnose.
U.S. Pat. No 3,556,439 (Autry, et al.) shows a triple flap leading edge device which has two pivoting portions (leading edge flaps 19, 20), plus a bullnose 21. In the landing position, both of the flap sections are rotated outwardly, with the forward of these extending forward of the other to form a slot. In the take-off configuration, one section rotates outwardly to the deployed position, and the other remains stowed within this.
U.S. Pat. No. 4,189,120 (Wang) shows a variable camber leading edge flap which is deployed by means of two links 30 and 36, this being similar in overall configuration to that shown in Cole et al. '870.
U.S. Pat. No. 4,189,121 (Harper, et al.) shows what is called a "variable twist leading edge flap". The flap extends downwardly and forwardly to its deployed position, and there is linkage which deploys a separate bullnose.
The following patents are of background interest only:
U.S. Pat. 4,351,502 (Statkus) shows a variable camber leading edge device for an airfoil, and does not involve a separately deployable flap.
U.S. Pat. No. 3,917,192 (Alvarez-Calderon) shows a mechanism having two separate flap members that deploy to form a double slotted arrangement.
German Patent No. 2,101,536 (Erelefeldt) shows a scissors type linkage which deploys a leading edge flap, this apparently being stowed against the outward surface of the wing for cruise.
Accordingly, there exists a need for a three-position variable camber Kreuger flap system having a linkage which will enable the deployed flap to move to first and second operative positions wherein the flap is ideally positioned and angulated for landing and take-off operations. Furthermore, there exists a need for such a system which provides the deployed, highly-cambered flap with sufficient support to impart stability thereto during such flight operations. Still further, there exists a need for such a system which is economical to manufacture and suitably low in weight, and Which can be mounted to the forward portion of an airplane wing without interfering with the installation of anti-icing ducting in this area.